Biochemical studies of the H-2 antigens of the surface membranes of L-5178Y thymoma have shown that additional peaks appear on SDS gels besides the normal 42,000 and 12,000 dalton components. The most prominent peak, also found on cell-free membrane preparations (MLP), is found in the 9,000 dalton region. A number of lymphocyte hybridomas have been isolated by hybridizing a myeloma cell line with spleen cells of a C57BL/10 mouse hyperimmunized with L-5178Y. Twelve of these hybridomas have been cloned and they continue to secrete anti-L-5178Y antibody. Four of the hybridomas are mu-producers, yet the antibody they secrete is not cytolytic for L-5178Y with either rabbit or guinea pig complement. The L-5178Y cells will grow as a solid tumor in DBA/2 mice when inoculated s.c. If the tumor cells are harvested more than 10 days after inoculation, the tumor cells have a greatly reduced capacity to bind H-2 antibody. An analysis of the products coded for the rat MHC has been carried out. It appears that there are at least 2 independent molecules coded for by the AgB.A locus and at least 2 independent sets of molecules coded for by the AgB.B locus. The spontaneous killer cell activity found in spleen cell cultures have been studied.